A tendon is a robust type of connective tissue composed primarily of collagen fibers that connects muscle to bone. Flexor tendons specifically facilitate flexion, which is the bending of a joint. While these structures exist throughout the body, the flexor tendons of the hand and wrist are frequently discussed due to their intricate nature and common susceptibility to injury. These specific tendons allow for the complex movements required for grasping, holding, and performing fine motor tasks.
Anatomical Structure and Location
Flexor tendons are composed mainly of dense, parallel bundles of type I collagen fibers, granting them high tensile strength. They originate from the flexor muscles situated in the forearm, which are responsible for generating the force necessary for movement. These long, cord-like structures then travel down the arm toward the wrist, passing through a confined space known as the carpal tunnel. The tendons are organized into two main groups: the flexor digitorum superficialis and the flexor digitorum profundus, which control the intermediate and distal joints of the fingers, respectively.
As the tendons enter the hand and fingers, they are encased within a protective structure called the tendon sheath. This sheath is a double-layered membrane that produces synovial fluid, a viscous substance that acts as a lubricant. The fluid minimizes friction as the tendon glides back and forth, allowing for smooth movement during finger flexion. This lubrication is important for preventing wear and tear.
Another distinguishing feature of the finger’s flexor tendon system is the series of fibrous bands known as the pulley system. These pulleys are strategically positioned along the length of the finger bones (phalanges). The system includes five annular pulleys, labeled A1 through A5, and three cruciform pulleys, labeled C1 through C3, which are thinner and criss-cross between the annular ones. These structures form a series of loops that physically secure the tendon close to the bone, which is a placement necessary for mechanical efficiency.
Mechanism of Movement
The process of bending a finger begins with a signal originating in the motor cortex of the brain, traveling down the nervous system to the forearm muscles. Upon receiving this electrical impulse, the muscle fibers shorten, or contract, generating mechanical force. This contraction pulls on the attached flexor tendon, effectively transmitting the muscle’s force across the wrist and into the hand. The result is the initiation of movement at the finger joints.
The tendon acts as a non-elastic rope, translating the linear pull of the muscle into angular motion at the joint. Specifically, the flexor digitorum profundus tendon is responsible for bending the distal interphalangeal joint, the one closest to the fingertip. Meanwhile, the flexor digitorum superficialis tendon primarily bends the proximal interphalangeal joint, located in the middle of the finger. The combined action of these two tendons allows for powerful and graded flexion across the entire finger.
The inherent strength and specific routing of the tendons allow for the precise gripping and fine motor control characteristic of the human hand. This action is constantly balanced by the extensor tendons, which run along the back of the hand and wrist. Extensor tendons perform the opposing function, pulling the joints straight to open the hand.
Understanding Flexor Tendon Injuries
The most frequent cause of flexor tendon damage is an acute laceration, often resulting from cuts by glass or sharp instruments. Because the tendons are superficially located on the palmar side of the hand and fingers, they are particularly vulnerable to penetrating trauma. A partial laceration may still allow some finger movement, but a complete severing of the tendon immediately disrupts the mechanical connection between the muscle and the bone. Injuries in the palm and finger, particularly in Zone II, pose significant surgical challenges due to the tight space and the need to preserve the delicate anatomy of the pulley system.
Beyond sharp cuts, flexor tendons can also suffer ruptures, which are complete tears caused by excessive force or chronic degeneration. A specific type of rupture is an avulsion injury, commonly known as Jersey Finger, which typically affects the ring finger. This occurs when the flexor digitorum profundus tendon is forcefully pulled away from its attachment point on the fingertip bone, sometimes carrying a small fragment of bone with it. This specific trauma prevents the individual from actively bending the joint closest to the fingernail.
The immediate and most telling clinical sign of a significant flexor tendon injury is the inability to actively bend the affected joint. The muscle is still capable of contracting, but the force is no longer transmitted to the skeleton, resulting in a loss of active flexion. Patients experience pain, swelling, and sometimes a visible or palpable gap where the tendon has separated.
A complication following a complete tear is tendon retraction, where the severed ends pull apart due to the constant tension exerted by the forearm muscles. The muscle belly contracts, pulling the proximal end of the tendon away from the injury site, often up into the palm or forearm. Because the ends are separated by a gap, the tendon cannot bridge the distance. Surgical intervention is required in most cases to retrieve the retracted end and rejoin the two segments.